This invention relates to an installation for the manufacture of ceramic products, in particular for the manufacture of ceramic sanitaryware.
As is well known, ceramic sanitaryware (such as washbasins, toilet bowls, bidets, shower trays and the like) is made by casting a fluid mixture (known as “slip” in the jargon of the trade, consisting of a ceramic body in aqueous suspension) in customary molds with a porous structure, made in particular from resins.
These porous molds are composed of at least two parts (usually known as “male” and “female” in the jargon of the trade) which are joined to form an internal cavity where the ceramic product is formed.
The porous surfaces that form the sides of each cavity will hereinafter be referred to as the inside surfaces of the mold.
Each mold part also comprises a rear outside surface or back, on which the forces necessary to keep the two parts together during the casting cycle are exerted, and a lateral outside surface.
At least one of the two outside surfaces is associated with auxiliary elements designed to support and keep the mold in place within the installation.
There are also contact surfaces which generally act as transitions between the lateral outside surfaces and the closing surfaces.
Internally, these porous molds are provided with a drainage system designed to allow the fluids that go through the inside surfaces to be channeled to the outside, or to pump fluids in under pressure in the opposite direction in order to detach the molded product from the mold walls or to recondition the mold part.
The two or more parts of the mold are mounted in suitable installations (that differ according to the type of product to be cast) and comprising at least the following:
a fixed structure which, through passive connecting means, acts as a support for at least one part of the mold;
drive means for moving and positioning at least one part of the mold at least in order to move the mold parts towards each other (so as to close the mold when casting is in progress) and away from each other to allow the cast piece to be extracted;
clamping means for keeping the mold parts in the correctly closed position, overcoming the forces generated inside the cavity during the casting cycle;
cavity service means such as means for feeding the slip into the mold when the mold parts are clamped shut or for injecting air for consolidating the slip and draining out the excess slip during the casting cycle;
service means for the above mentioned drainage system.
Further, one of the well known characteristics of porous resin molds is their good mechanical strength which allows them to be used for high pressure casting, that is to say, for pumping the slip into the mold and subsequently forming the cast wall thickness at high pressure (usually between 3 and 15 bar).
These pressures inside the mold, however, produce forces in directions normal to the inside surfaces of the mold parts, with the risk of deforming the mold: the directions adopted by the force components are not only the direction in which the mold parts are moved together and clamped shut but also the directions at right angles (and hence transversal) to the mold part clamping direction.
These forces must therefore be opposed by suitable devices in order to “contain” the forces in play.
As regards the forces generated in the mold clamping direction, the above mentioned casting installations may (in one prior art solution) comprises a fixed abutment wall operating on the back of one of the mold parts, and a drive cylinder that operates on a mobile wall which in turn operates on the back of the other part of the mold.
As is also known in the trade, the cylinder may apply on the mobile part of the mold force that is constant or variable instant by instant as a function of slip pressure (known in the jargon of the trade as “proportional clamping”). To this must be added the fact that the relative movements of the mold parts towards each other during the casting cycle may be either free and, hence, determined only by the balance of the forces in play and by the deformability characteristics of the resins, or limited to a maximum value thanks to the presence of mechanical stops which absorb the force applied by the piston in excess of the force sufficient to determine the maximum acceptable deformation (known in the jargon of the trade as “controlled deformation clamping”).
As regards the forces generated in directions at right angles to the clamping direction, on the other hand, prior art solutions include purely passive mechanical systems that can preload the resin to varying extents with initial compression stresses along said transversal directions and whose reaction to the forces produced inside the cavity during the casting cycle and tending to compress the mold walls and to deform the lateral outside surface towards the containment device depends only on the rigidity of the containment device itself, or active mechanical systems where the reaction of the containment device is controlled over time and as a function, instant by instant, of slip pressure.
For this purpose, the Applicant has devised and produced a device for “containing” the forces (see also patent EP 1.043.132) where one of the half-parts of the mold comprises a frame that delimits a space, between the frame and the half-part, for housing an element expandable by a fluid from the outside and designed to contain the forces generated by the pressure of the slip inside the mold.
This system regulates the pressure of the fluid inside the expandable element, which is correlated constantly with slip pressure, obtaining an improved reaction, eliminating the potential deformation of the mold which leads to undesirable stress on the part being cast and, hence, to possible defects, and controlling elastic contractions of the mold.
In view of the excellent results obtained by this solution in controlling the components of the forces in the directions at right angles (and hence transversal) to the mold part clamping direction, it would be desirable to also be able to control the force in the clamping direction more effectively than has been possible up to now.
At present, the solution involving a proportional force of the clamping cylinder is calibrated in such a way as to apply to all the parts of the mold the same pressure as that applied by the slip.
The crux of the matter, however, is that the hydraulic system, which acts on a flat part which is rigid by its very nature is a system with limited opposing precision, that is to say, with relatively wide tolerances compared to the requirements of the mold and with a considerably lower precision than that of the fluid system for the other components, which adapts the movements of the abutment surface opposing the lateral outside surface of the mold to the compressibility requirements of the resin layer below.